Ice storage air conditioning

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Ice storage air conditioning is the process of using ice for thermal energy storage. The process can reduce energy used for cooling during times of peak electrical demand.^[1] Alternative power sources such as solar can also use the technology to store energy for later use.^[1] This is practical because of water's large heat of fusion: one metric ton of water (one cubic metre) can store 334 megajoules (MJ) (317,000 BTU) of energy, equivalent to 93 kWh (26.4 ton-hours).^[2]

The original definition of a "ton of cooling capacity" (heat flow) was the heat needed to melt one ton of ice in a 24-hour period. This heat flow is what one would expect in a 3,000-square-foot (280 m²) house in Boston in the summer. This definition has since been replaced by less-archaic units: one ton of HVAC or refrigeration capacity is approximately equivalent to 3520 Watts. A small storage facility can hold enough ice to cool a large building from one day to one week, whether that ice is produced by anhydrous ammonia chillers.

Ground freezing can also be utilized; this may be done in ice form where the ground is saturated. Systems will also work with pure rock. Wherever ice forms, the ice formation's heat of fusion is not used, as the ice remains solid throughout the process. The method based on ground freezing is widely used for mining and tunneling to solidify unstable ground during excavations. The ground is frozen using bore holes with concentric pipes that carry brine from a chiller at the surface. Cold is extracted in a similar way using brine and used in the same way as for conventional ice storage, normally with a brine-to-liquid heat exchanger, to bring the working temperatures up to usable levels at higher volumes. The frozen ground can stay cold for months or longer, allowing cold storage for extended periods at negligible structure cost.^[3][4]

Replacing existing air conditioning systems with ice storage offers a cost-effective energy storage method, enabling surplus wind energy and other such intermittent energy sources to be stored for use in chilling at a later time, possibly months later.